Spectroscopic measurements of the disc around a black hole (artist's impression)

Scientists use spectroscopy — a technique that breaks light up into its component wavelengths — to study the vicinity of supermassive black holes. As matter spins around the black hole, the Doppler effect means that one side appears slightly redder, and the other slightly bluer than it really is. This is because the wavelength of light is squashed towards the shorter blue wavelengths on the side of the disc moving towards us, while it is stretched towards the longer red wavelengths on the side moving away. Note that the effect is exaggerated in this computer simulation, which depicts the vicinity of the black hole in the galaxy Messier 87 — one of the first to be studied by Hubble.